Case Study : Teenager with Knee Pain

First of all the question is  how we can localize the salient abnormality ?


T2W versions of essentailly the same images:




Here are coronal STIR images of same knee:



Findings :


There is an abnormally shaped (discoid) lateral meniscus. There is increased signal in the posterior aspect of the lateral meniscus abutting meniscocapsular attachment.


Diagnosis:


Discoid meniscus with posterior tear.


Discussion:

A discoid meniscus is a congenitally enlarged meniscus that has lost its normal semilunar C shape. It is due to the body's failure to resorb the central portion of the meniscus during development. The lateral compartment of the knee is more commonly affected. A discoid meniscus is more prone to tear and patients will present with the usual symptoms of a torn meniscus such as pain, clicking and locking of the knee.

Radiologic overview of the diagnosis:

A discoid meniscus is best evaluated through MR imaging. The discoid meniscus will demonstrate normal meniscal signal which is hypo intense MR signal on T1 and T2 weighted images. The meniscus will usually measure greater than 13mm on cross section and show greater than 3 body segments on consecutive sagittal 4mm thick images. The meniscus will appear as a pancake spanning from the periphery to the intercondylar notch.

When torn, the meniscus will demonstrate irregular margins and increased signal intensity on T1 and proton density images. In this case, there is a discoid meniscus in the lateral compartment of the knee. There is a tear within the posterior horn of the meniscus.

Informational and Key Points :

• A discoid meniscus is a congenitally enlarged meniscus that is more prone to tear.
• A discoid meniscus is best evaluated with MR and will have the same signal as a normal meniscus on T1 and T2 weighted sequences.
• It will usually measure greater than 13 mm in cross section and will appear as a pancake spanning the entire compartment.
• Meniscal tears will appear as increased signal on T1 and proton density images.

 

Courtesy of  Dr. Wei-Shin Wang,
Indiana University Department of Radiology,
for contributing this case.